Semiconductor manufacturing involves various steps involving forming a layer of material, patterning the material, forming another layer of material and so on. Metrology is a key component of process control for any manufacturing process. In the case of semiconductor manufacturing, metrology is used in a variety of capacities including the measurement of a thickness of a deposited material layer.
One method of measuring a film is to cut (e.g., scribe and break) the substrate through a feature of interest, thus exposing a cross-section of the feature. The feature may then be examined using microscopy, for example, scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Unfortunately, the substrate is destroyed by cutting. Moreover, the number of features that can be examined are limited to those features lying along the cut.
Another method includes using a measuring instrument set up to shine a beam of light through the material to be measured, and using the index of refraction of material, calculating a film thickness. A problem with this method is that the measurement needs to be done for a material that stops on a different material having a different index of refraction, for instance, silicon on silicon dioxide or vice-versa.
Another method that lends itself to in-line metrology includes creating a proxy in the form of features that are intended to be representative of one or more dies. The proxies may be placed between selected dies on the substrate. The features in the proxy may be measured during various steps in the manufacturing process using a probe such as a light beam having small spot size about the size of the proxy structure. However, it can be difficult to align the substrate and position the proxies within such a small spot size. Also, depending on the layer grown and proxies used for metrology, features in the proxies may or may not be representative of features in the dies.